Effect of Soft–Hard Rock Composites on Rock Breaking Efficiency of TBM Disc Cutters in Deep Tunnels

Soft–hard composite strata are widely distributed in the surrounding rock of deep tunnels, which severely reduces TBM excavation efficiency. To elucidate the rock-breaking mechanism of TBM disc cutters in composite strata and to address unresolved issues related to cutter force evolution, a self-developed rotary cutting test platform was employed, and three types of large-scale samples (red sandstone, granite, and red sandstone–granite composites) were prepared, on which systematic rotary rock-cutting experiments were conducted under varying confining pressures, rotational speeds, and penetration depths. The results indicate that rock failure in composite strata exhibits pronounced heterogeneity, with significant stress concentration occurring at soft–hard rock interfaces, leading to abrupt increases in normal force and torque. Penetration depth is the most sensitive factor influencing cutting force and specific energy, followed by confining pressure and rotational speed. The minimum specific energy and maximum rock-breaking efficiency are achieved at a penetration depth of 2.5 mm, a confining pressure of 7 MPa, and a rotational speed of 2.5–3 r/min. Furthermore, a dynamic model describing the evolution of disc cutter normal force and torque at soft–hard rock interfaces was derived based on the CSM theoretical framework, and its validity was verified using the experimental results. Integrating experimental observations with theoretical analysis reveals that rock fragmentation in composite strata is dominated by radial tensile cracking in hard rock and shear-dominated crushing in soft rock, while strong stress perturbations and coupled failure occur at the composite interface. This study clarifies the force evolution and fracture mechanisms of disc cutters operating in composite strata and establishes a reliable dynamic prediction model for cutter loads, providing theoretical support and engineering guidance for TBM parameter optimization and cutterhead design.